In addition to incremental signals with respect to the relative displacement of two objects movable relative to each other, conventional position-measuring devices often supply so-called reference-pulse signals as well. Given a defined relative position of the objects movable relative to each other, an exact absolute reference of the position measurement may be produced based on these signals. To generate the reference-pulse signals, fields having reference markings are arranged on sides of the scale of the position-measuring device at one or more locations. A number of conventional possibilities exist regarding the arrangement of the reference markings on the scale.
Thus, for example, in U.S. Pat. No. 4,263,506, it is described to arrange the reference markings on the scale laterally adjacent to the scale-division track having the incremental graduation. The problem in that case, however, is that in the event the scale and scanning unit twist about an axis perpendicular to the scale plane or scanning plane, as a rule, the exact allocation of the reference-pulse signal to a specific period of the incremental signals is no longer ensured.
In addition, it is also possible to integrate the reference markings directly into the track having the incremental graduation, as is described, for instance, in U.S. Pat. No. 3,985,448, and to then derive the reference-pulse signal directly from the incremental graduation. Such a system offers special advantages with respect to the allocation of the reference-pulse signal to the incremental signals, since even if the scale and scanning unit twist as mentioned above, the correct allocation always remains ensured. For example, one or more omitted bars or marks of the incremental graduation may be used as reference marking at the desired reference position on the scale in the incremental graduation, that is to say, the track having the incremental graduation has a discontinuity with respect to an optical property at one or more defined locations, in order to produce the reference-pulse signal.
Further variants regarding the integration of reference markings into the track having the incremental graduation are also described in German Published Patent Application No. 35 36 466 or U.S. Pat. No. 4,866,269. These documents propose forming the reference markings as a periodic mark sequences or bar sequences in the incremental graduation, or else to utilize regions having altered optical properties as reference markings which differ from the remaining incremental graduation.
Basically, however, with the integration of reference markings into the incremental graduation, it may be problematic that the periodic incremental signal is also thereby disturbed at this location, since one is forced to ensure sufficient detection reliability for the reference-pulse signal. Particularly if a high interpolation of the incremental signal is desired, which in turn assumes the best possible signal quality of the same, difficulties result when the incremental signal at the reference position deviates markedly from an ideal signal form.
Furthermore, in German Published Patent Application No. 41 11 873, it is described to generate a reference-pulse signal from a track that is arranged on the scale merely for generating region information, i.e., to identify precisely on what side of a scale-reference position a scanning unit is located. At the junction between two different regions, a reference-pulse signal can also be generated from the corresponding region signals. However, in this case, the correct detection of a reference-pulse signal at this location is problematic.